If a device has a passive output, then you need an external power supply to support it. If the device is active, then you already have an output signal with the power supply, so you don’t need another one.
Two-wire transmitters are passive because they need external power to close the loop before you can read the signal. But four-wire devices have different scenarios.
Active vs passive devices
Active and passive devices provide different outputs. But some transmitters also come with features that allow you to select whether the device provides an active or passive output.
Four-wire vs two-wire transmitters
Four-wire transmitters have power supplies separate from the output. So if the output is active, then you have the signal and the power supply. But if the device is passive, then the control system should provide the power to close the loop and send the output.
One more thing – if the device has an active output, the input of the control system should be passive. But if the device is passive, the input should be active to provide power. Make sense? If not, drop me a comment and I’ll help out!
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